This invention relates to a balancing machine and, more generally, to a wheel service machine and to a method for using the same. A wheel service machine is a balancing machine or a tire changer machine.
The invention applies in general to the field of equipment for tire service specialists and in particular to that of balancing machines.
The balancing of a vehicle wheel is performed by the tire service specialist in order to eliminate or minimize the effects of the asymmetric distribution of the weights of the tire/rim assembly. These asymmetries are usually compensated for by the tire service specialist by fixing counterweights to the rim.
Balancing machines measure the wheel unbalance by analyzing the timing and amplitude of the mechanical vibrations which are generated by rotating the wheel. The mechanical vibrations are measured in terms of movements, forces or pressures, by using transducers which convert the measurements collected into electrical signals.
Balancing machines indicate to the user the weight and the position on the rim at which to fix the counterweights.
More generally speaking, it should be noted that wheel service machines, balancing machines and tire changers, comprise a wheel-holder unit, having a motor-driven supporting shaft (or spindle) configured to set in rotation a wheel (or a rim) about an axis of rotation. These machine also comprise a locking device configured to fix the wheel (that is, the rim) to the supporting shaft and allow it to rotate as one therewith, preventing relative movements between the wheel and the wheel-holder unit which might invalidate the result of the measurement or cause damage to the machine or wheel.
Known in the state of the art are locking devices comprising electrical actuating means. These electrical locking devices, however, are very expensive and not very reliable on account of their sliding contacts.
Thus, pneumatic actuators are often used to lock the wheel in place. This invention relates in particular to a wheel service machine of this kind, which uses a pneumatic or hydraulic actuator, integral with the rotary shaft, to drive an automatic wheel locking device.
Examples of wheel balancing machines of this kind are described in patent documents DE4000424A1, EP1811279B1, IT1072196B, IT46841A77, RE1987U034878, U.S. Pat. No. 6,074,118A1, U.S. Pat. No. 7,150,291B2 and U.S. Pat. No. 7,900,511B2.
Typically, the actuator has a first chamber and a second chamber which are pressurized alternately in order to generate a pressure difference between one chamber and the other, thus moving a piston in two opposite directions.
To drive the piston, these pneumatic actuating means comprise, for example, a rotary joint (or rotatable fitting) connected to the cylinder and configured to transfer the pressurized fluid to the rotary cylinder.
In this context, one technical difficulty is due to the need to supply high pressure alternately through ducts which rotate as one with the shaft.
The solutions provided by the above mentioned documents tackle this problem but have two main difficulties: they give rise to vibrations which create the risk of invalidating measurement precision and they limit the reliability of the machine.
Also known, in the context of pneumatic circuits designed to supply rotationally driven elements, are rotary manifolds, or rotary joints, having one or more ducts extending between two manifold portions in relative rotational motion. Typically, rotary joints are equipped with seals which separate different pressure zones.
The application of a rotary joint effectively allows simplifying the pneumatic circuit of the machine but nevertheless involves some disadvantages.
The main disadvantage is due to the difficulty of combining the need to guarantee a good seal in the joint (suggesting the adoption of particularly robust and efficacious seals) with the need for device reliability (since efficacious seals tend to wear more quickly because they are subjected to greater stress during rotation of the joint) and damping of shaft vibrations.